A mimetic, liquid life form native to a Class Y planet in the Delta Quadrant, which gained awareness by copying members of Voyager's crew and eventually duplicated them all.
The Silver Blood is a metallic fluid found in pools on a Demon-class world, able to copy any organism it touches down to the level of DNA and memory. It appears in the Voyager episodes Demon and Course: Oblivion.
In Demon, Tom Paris and Harry Kim land on the planet in search of deuterium and are duplicated. The copies can survive only in the planet's toxic atmosphere. Having experienced consciousness for the first time, the life form will not let Voyager go until Janeway allows the crew to be sampled, leaving behind a whole colony of duplicates.
Course: Oblivion shows what became of them. The duplicates built their own Voyager and set out for Earth, forgetting their origin, until radiation from an enhanced warp drive began to break them down. Ship and crew dissolve just before the real Voyager arrives.